Humans are unique among social creatures in their willingness to bear personal costs to punish those who have harmed others. Despite the centrality of such third-party punishment decisions to modern institutions of justice, researchers are still unclear as to how the brain combines evidence of intentionality and harm and how humans make these judgments.

Now, a study from researchers at Vanderbilt University and Harvard University reveals new insights into the human’s unparalleled sense of justice, specifically, the precise role of the dorsolateral prefrontal cortex (DLPFC), one of the most recently evolved regions in the human brain. The team state that the findings reveal that DLPFC integrates information about a suspect’s blameworthiness for wrongful acts and the resulting harm to others, enabling the person to decide on the appropriate level of punishment. The opensource study is published in the journal Neuron.

Previous studies show that the success of the human race is thought to rely largely on their capacity for large-scale cooperation; this, in turn, hinges on the uniquely human ability to establish and enforce social norms. To make decisions about how to punish those who violate these norms, it’s necessary to integrate information about a suspect’s culpability as well as the harmful consequences of the transgression. The DLPFC is well positioned to play this role; its cellular organization and high level of connectivity with other brain regions has been shown to make it specialized for integrating multiple streams of information in order to select appropriate responses. Researchers have shown that the DLPFC performs this integrative role in non-social cognitive tasks, and the DLPFC appears to be activated in many studies of moral and legal norm-based decision-making. However, the precise role of DLPFC in making these judgments has been unclear.

The current study used repetitive transcranial magnetic stimulation (rTMS), a noninvasive way of stimulating the brain using magnetic fields, and functional magnetic resonance imaging (fMRI) in human subjects who made blameworthiness judgments and punishment decisions about a series of crime scenarios. Subjects were shown a short written scenario describing a protagonist called John committing a crime, ranging from simple theft to assault and murder. In some cases, the crime was deliberate and John was fully responsible for his actions and in other instances, his culpability was diminished due to duress, psychosis, or other mitigating factors. In separate sessions, subjects either rated John’s blameworthiness or the severity of punishment he deserved.

The researchers first used rTMS to magnetically stimulate, and thereby temporarily disrupt, the DLPFC activity in 66 healthy volunteers. Half of the participants received active rTMS and the other half received placebo or ‘sham’ rTMS. Results show that DLPFC disruption reduced the level of punishment for wrongful acts without affecting blameworthiness ratings, suggesting that these two aspects of norm-based judgments rely on distinct cognitive and neurobiological processes.

Data findings show that rTMS only lowered punishment ratings when John’s actions were deliberate but resulted in minimal harm. Further analysis revealed that DLPFC disruption caused subjects to base their punishment decisions more on the consequences of the crime rather than on John’s intentions. The findings suggest that the DLPFC plays a critical role in balancing information about intent and harm to enable appropriate punishment decisions.

A separate brain imaging experiment in the same study corroborated the main rTMS findings with results showing that there was greater activity in the DLPFC during punishment decisions compared to blameworthiness judgments. The lab note that DLPFC activation was sensitive to John’s culpability level, however, this effect was only found for punishment and not blameworthiness judgments. Therefore, the researchers state that the findings suggest that the DLPFC is not involved in assessing culpability per se; rather, this brain region uses information about culpability specifically to support punishment decision-making. The group conclude that taken together with past results, the findings suggest that the DLPFC receives relevant information about culpability and harmful consequences from other brain regions and then integrates this information to support punishment decision making.

The team surmise that while this study does provide new insight into how human brains make decisions on punishments, the effects reported are modest in size. They go on to add that the value of their study lies in its ability to reveal the basic mechanisms of norm-enforcement decisions and a region of the brain unique to humans. For the future, the researchers now plan to identify the precise computations involved in this integrative process.